1. Field of the Invention
The present invention relates to photodiodes reverse-biased to a voltage slightly greater than their avalanche threshold. Such photodiodes thus operate in so-called Geiger mode and enable to detect radiations of very low light intensity. Such photodiodes are commonly called SPADs (“Single Photon Avalanche Diodes”) and are especially used for the detection of single photons and the counting of photons.
2. Discussion of Prior Art
FIG. 1 is a cross-section view schematically showing a simplified example of a structure of a photodiode intended to operate in avalanche and that can be used for the detection of single photons. The photodiode is intended to be illuminated on its upper surface 2.
In the upper P-type portion of a semiconductor substrate 1, an N-type ring 3 extending from the upper surface of the substrate surrounds a P-type well 5. An N-type buried layer 7 extends under P-type well 5. The P-N junction causing the avalanche phenomenon corresponds to junction 8 between P-type well 5 and N-type buried layer 7.
A P-type region 9, more heavily doped than well 5, is formed at the center of well 5 and corresponds to the anode contact of the photodiode.
N-type regions 11, more heavily doped than ring 3, extend in ring 3 from the upper surface of the substrate. Regions 11 are for example distributed in ring 3 at regular intervals and correspond to the cathode contact of the photodiode.
FIG. 2 shows a conventional characteristic I(V) 13 of the current versus the voltage of a diode.
For an operation as a single-photon detector, the photodiode is reverse-biased to a voltage Vpol slightly greater than avalanche threshold Va of the photodiode. Call Ve the voltage difference between bias voltage Vpol and avalanche threshold Va. The operating point of the photodiode when it absorbs no photons corresponds to point 14 of characteristic I(V) 13. Current I1 crossing the photodiode is almost non-existent.
As soon as a photon is absorbed by the photodiode, the electron of the generated electron-hole pair triggers an avalanche phenomenon which makes the photodiode transit from operating point 14 to operating point 15 (current I2) of characteristic I(V) 13. Current pulse I2 is greater than current I1 by several orders of magnitude.
FIG. 3 is an equivalent electric diagram of an example of a detection circuit comprising a photodiode 21 such as that illustrated in FIG. 1 and enabling to detect single photons.
Anode 9 of photodiode 21 is connected to a power supply terminal 25 at voltage −Va. Cathode 11 of photodiode 21 is connected, via a resistor Rq, called quenching resistor, to a power supply terminal 27 at voltage Ve. A comparator 29 is also connected to cathode 11 of photodiode 21.
The single-photon detection circuit illustrated in FIG. 3 operates as follows.
As soon as no photon is absorbed by photodiode 21 reverse-biased to voltage Vpol=Va+Ve slightly greater than the avalanche threshold, current I1 running through the photodiode is very low. The voltage at node E is almost equal to Ve and the output of comparator 29 is at a low level.
When a photon is absorbed by the photodiode, it triggers an avalanche phenomenon. The current crossing photodiode 21 increases rapidly. A voltage drop appears across resistor Rq and the voltage of node E drops. The output of comparator 29 then switches from the low level to the high level. Resistor Rq further enables to quench the avalanche phenomenon triggered by this photon to be able to detect the absorption of another photon.
FIG. 4 is a top view of an example of an integrated embodiment of an avalanche photodiode assembly enabling to detect single photons, comparator 29 being shown in the form of an electric symbol. The elements of FIG. 4 common with those of FIGS. 1 and 3 are designated with the same reference numerals.
Only the metallizations of a photodiode 21 are shown in FIG. 4. A metallization 31 forms a contact on anode 9 of the photodiode, and a metallization 33 forms a contact on cathode 11 of the photodiode.
Anode 31 of the photodiode is connected to a conductive track 35. A portion 37 of conductive track 35 is located above the photodiode. Conductive track 35 is connected to another photodiode, not shown, by a conductive track 36. A conductive track 39 connects cathode 33 of the photodiode to resistor Rq, and to comparator 29.
To mask the photodiode as little as possible, portion 37 of anode conductive track 35 is as narrow as possible.
Conductive track 35 normally does not conduct high currents, even during avalanches, since resistor Rq, limits the current. The decreased width of portion 37 of this conductive track thus apparently raises no issue.